1. Technical Field
The present invention relates to a test system for simulating impairments, including losses, errors, noise and jitter, in a network wireless communication signal to enable estimation of the resulting degradation in voice or video quality.
2. Related Art
Operators need to ensure that their systems provide excellent multimedia quality. Every time a new handset is introduced, it should be tested to make sure it produces clear audio and video under ideal and under degraded coverage conditions.
FIG. 1 illustrates the classic test system to measure media quality in a wireless system. Typically the media quality is measured or estimated for voice media or for video media. As shown, the system includes two User Equipment (UE) devices 2 and 4 which enable telephony type voice communications over a wireless link. The devices 2 and 4 can be cellular mobile phones. The UE 2 is used by a speaker to provide a voice reference media 1 that is converted by the UE 2 to a packet data signal and transmitted over a wireless air interface link 6 to a wireless system 8. The wireless air interface link 6 is part of a first Radio Access Network (RAN) and can carry mobile phone signals such as LTE, UMTS, CDMA or GSM signals. The wireless system 8 can include a base station for mobile phone communications. The wireless system 8 then communicates the packet data signal again through another wireless interface link 10 of a second RAN to another UE 4. The UE 4 is also the Device Under Test (DUT) as it converts the packet data signal back to an audio signal that is provided through a speaker of the DUT UE 4 for listeners to hear. The audio signal played through the speaker provides a degraded media signal 11 to listeners. The listeners then determine the quality of the degraded media signal.
Voice quality of a connection can be measured and reported in many ways. Historically the preferred method was to let a panel of listeners, as illustrated in FIG. 1, evaluate the perceived received quality of the audio received from one or more speakers. The resulting scores were averaged and captured as a Mean Opinion Score (MOS). The MOS scale ranges from 1 (bad) to 5 (excellent). The score for a wireless connection depends on the codec, or signal encoding and decoding method that is used. The score also strongly depends on the latency and reliability of the air interfaces 6 and 10. For instance GSM has a value of 3.5 and AMR-WB has a value of 4.2.
Evaluating a MOS with real listeners is subjective and a large number of listeners must be used. Gathering people to listen is time consuming and costly. In recent years more objective methods have been developed to measure the MOS. For these methods one injects reference audio from a source file (the ‘reference file’) recorded from a speaker and then captures the resulting audio after transmission through at least one RAN in a target file (the ‘degraded’ file.) One can then use software to compare and analyze the reference file and the degraded file to estimate the MOS.
Several software packages are commercially available for automated assessment of speech quality and to provide a perceptual objective listening quality assessment. Example software packages are PESQ and POLQA. PESQ stands for “Perceptual Evaluation of Speech Quality.” It is standardized as ITU-T recommendation P.862. POLQA stands for “Perceptual Objective Listening Quality Assessment” and provides automated assessment of speech quality. It is standardized as ITU-T recommendation P.863.
Voice quality strongly depends on the properties of the Radio Access Networks (RANs) that are being used by the source UE and the target UE. The components making up a RAN (e.g. the source UE and base station) and the air interface that connects them (e.g. the LTE air interface) introduces impairments such as packet losses, packet delays, fluctuations in the packet losses (jitter) and packet errors (frame errors). The RAN may be a RAN of a wide area wireless network that uses GSM, UMTS, GPRS, CDMA or LTE and the like, or the RAN of a local area wireless network such as DECT, Bluetooth, and Wi-Fi and the like. Another contribution comes from the internal components of the network that interconnects the source RAN and the target RAN, as internal components in the wireless system 8 in FIG. 1. For simplicity these internal components are not shown but may include well-known entities such as one or more base stations (such as LTE Node-Bs), mobile switching centers, regional network controllers, serving and packet gateways, gateway controllers, mobility management entities, the various Call Session Control Functions (CSCFs) of an IP multimedia system such as the Proxy-CSCF, the Interrogating CSCF, and the Serving CSCF and various databases. The wireless system 8 may further contain entities that manage the quality of service, such as a policy charging and rules function.
FIG. 2 shows components used in conventional test systems that emulate the effect of impairments to enable evaluation of one or more RANs in a laboratory environment. The emulation test components of FIG. 2 are provided in the test system 20 which receives signals from UEs in a system otherwise similar to FIG. 1. The signals transmitted to and from the test system 20 include a reference media signal 1 from the UE 2 and the output includes a degraded media signal 11 provided from a DUT UE 4. Components carried forward from FIG. 1, as well as components carried forward in subsequent drawings, are similarly labeled.
The test system 20 includes faders 22 and 28 and components 24 and 26 that emulate two separate RANs 24 and 26. A fader is a device that emulates the behaviors of an air interface, for example by varying the signal strength of the modulation of the uplink and/or downlink air interface connections. The test system 20 provides a way to produce artificial impairments of a source RAN and a target RAN by emulating each RAN with a signaling tester (like an Anritsu MD8430), and by imposing artificial impairments on each air interface with a fader (like an Anritsu MF6900A.)
To estimate a MOS using the test system 20 of FIG. 2, one configures the testers and the faders 22 and 28 according to specific RAN parameters. This causes precisely controlled losses, delays, jitter and frame errors on the air interfaces. Next a call is started between the source UE 2 and the target UE 4 and a user plays the sound from a reference media file 1 into the source UE 2, for example via the source UE 2 built-in microphone or via the source UE 2 microphone jack. The sound is then captured at the target UE 4 from its built-in speaker or headset jack, and converted to digital data and stored as a degraded media file 11. PESQ or POLQA is finally used to compare and analyze the files and to obtain the MOS. Note that the same system in a slightly different configuration may be used to obtain a MOS for multimedia transmission from the DUT UE 4 to the peer UE 2.
Operators need to measure the impact on the MOS of the various parameters that control the air interface so that they can optimize throughput without degrading voice quality. What is needed is a method that can automatically evaluate the MOS for a UE for a voice call that involves a source RAN and a target RAN under various RAN conditions without the cost of expensive equipment such as the faders in shown in FIG. 2.